This invention is related to diblock, triblock, higher multiblock and star-block copolymers of a polyorganosiloxane and a polyamide and the reagents and methods used to prepare them. Diblock copolymers have two chemically different polymers covalently bonded to each other at one end of their chains. Triblock copolymers have a central polymeric block covalently bonded at its two chain ends to two different polymeric blocks (which may be the same or different chemically). Higher multiblock copolymers consist of multiple polymer blocks of chemically different repeat units covalently bonded to each other through their chain end(s). And star-block copolymers are comprised of a central plurifunctional core with three or more radial arms where each arm can represent a diblock, triblock or multiblock copolymer. For this invention, the polyorganosiloxane may have alkyl, aryl, alkenyl or a combination of these substituents on silicon and examples include polydimethylsiloxane, polydiethylsiloxane, polymethylphenylsiloxane and polydimethyl-comethylvinylsiloxane. The polyamide may be selected from polyamide-4, polyamide-5, polyamide-6, up to polyamide-14, or any combination of these, with polyamide-6 being preferred.
Polyorganosiloxane-polyamide block copolymers may be prepared by the use of functionalized polyorganosiloxanes commonly called macroinitiators. For the anionic copolymerization with cyclic lactams, polyorganosiloxane macroinitiators are comprised of N-acyllactam groups at the end(s) of the polyorganosiloxane chain which are used to initiate lactam ring opening to yield the polyamide-n blocks, where n=3, 4, 5, . . . 14.
Previous attempts at preparation of polydimethylsiloxane-polyamide-6 block copolymers via the macroinitiator method (M. J. Owen, et.al., Br. Polym. Jnl., 4, 297 (1972); P. M. Lefebvre, et.al., Makromol. Chem., 183, 2453 (1982); P. P. Policastro, et.al., Polym. Bull., 16, 43 (1986)) have resulted in copolymers with very low block molecular weights. These low molecular weight copolymers are unsuitable for impact modification, fibers and other toughening applications because of poor mechanical properties such as lower tensile, shear and yield strengths as well as the inability to emulsify blends with homopolyorganosiloxane and/or homopolyamide. In order for a block (or graft) copolymer to emulsify a blend of the two respective homopolymers, the block molecular weights must be greater than the molecular weight of the respective homopolymers; otherwise, phase separation (separate aggregation of the homopolymer(s) and copolymer) occurs and the material properties are not sufficiently enhanced.
For the previously prepared polydimethylsiloxane-polyamide-6 block copolymers, short polyamide-6 blocks resulted from: 1) a very high concentration of macroinitiators during copolymerization such that the .epsilon.-caprolactam was distributed among many competing dimethylsiloxane blocks; or 2) from copolymerization of the .epsilon.-caprolactam at low temperatures (about 75.degree.-130.degree. C.). At these temperatures, the solubility of polyamide-6 in .epsilon.-caprolactam is quite low so that the polymer precipitates from its monomer at very low conversion; only oligomeric (degree of polymerization, D.sub.p =10 or so) polyamide-6 is formed at temperatures of 75.degree. C. (O. Wichterle, Makromol. Chem., 35, 174 (1960)).
With the aforementioned polydimethylsiloxane-polyamide-6 block copolymers, short siloxane blocks resulted from the substantial depolymerization of the polydimethylsiloxane at temperatures as low as 110.degree. C. in the presence of conventional lithium and sodium caprolactam catalysts (P. M. Lefebvre, et.al., supra.). After 2 hours exposure of unfunctionalized polydimethylsiloxane in excess .epsilon.-caprolactam to lithium and sodium caprolactam separately at 110.degree. C., the polydimethylsiloxane molecular weight decreased by approximately 25% and 45%, respectively. During actual copolymerizations, the polydimethylsiloxane depolymerization is even greater and low diblock copolymer yields of only 8-10% were achieved using these catalysts at a temperature of 110.degree. C. (P. M. Lefebvre, et.al., supra.).
Other catalysts such as LiAlH.sub.4 were used (M. J. Owen, et.al., supra.) in copolymerizations of .omega.-N-(acyllactam)polydimethylsiloxane and .epsilon.-caprolactam or lauryllactam in toluene at 110.degree. C., but gave copolymers with very low molecular weight polyamide blocks. Although LiAlH.sub.4 causes only moderate rearrangement of the polydimethylsiloxane at 110.degree. C. in toluene and is less destructive than other catalysts, e.g. alkali metal caprolactams, the resulting low molecular weight copolymers were unsuitable for toughening applications.
From previous work, it is apparent that copolymerization temperatures of about 110.degree. C. produce copolymers with low molecular weight polyamide blocks. At higher temperatures (greater than about 150.degree. C.), where high molecular weight polyamides must be synthesized, the polyorganosiloxane depolymerization from the commonly used alkali metal lactams or LiAlH.sub.4 catalysts is much more substantial and copolymer yields are even poorer. At higher temperatures, the alkali metal lactam and LiAlH.sub.4 catalysts chemically alter the structure of the polydimethylsiloxane decreasing its molecular weight, increasing its polydispersity and creating cyclic dimethylsiloxanes from an initially linear polydimethylsiloxane. Quite simply, all of the conventional catalysts comprised of LiAlH.sub.4 and metal lactams (see R. M. Hedrick and J. M. Gabbert, U.S. Pat. No. 4,031,164, Jun. 21, 1977; assigned to Monsanto Co., St. Louis, Mo.) for the ring opening polymerization of cyclic lactams in the presence of polyorganosiloxane macroinitiators at elevated temperatures are simply too destructive towards the polyorganosiloxanes for the successful production of high molecular weight copolymers.